These studies will characterize a new observation that X-rays induce in mammalian cells a persistent hypermutability to subsequent exposure to DNA-damaging agents. This phenomenon was discovered by mimicking a sequence of therapeutic treatments that produces high cancer rates in exposed human beings. This phenomenon of induced hypermutability has several characteristics which raise concern that it may be important in human exposure to X-rays: it is induced at high level; it persists in the progeny of treated cells; it is produced by multiple small fractions; and it may result from only partially reparable damage. The work in this grant will evaluate these several characteristics in human and rodent cells, and determine the relationship between hypermutability and better characterized effects of ionizing radiation such as cytotoxicity and specific locus mutation. Experiments will determine whether X-ray-induced damage can be repaired between fractionated-dose exposure and during the quiescence state in growth-arrested cells. X-ray treatments which induce and express hypermutability in mammalian cells will be evaluated in culture systems which transform to malignant growth in vitro to determine whether hypermutability and transformation share common mechanisms.